1. Field of the Invention
The present invention relates to a charged-particle apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions of an observed area on a sample surface. Hence, the apparatus can be used to inspect and/or review defects on wafers/masks with high resolution and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on surfaces of wafers/masks during fabrication processes, which reduce the yield to a great degree. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Accordingly, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beam employ the principle of scanning electron microscope (SEM) with a single electron beam, which therefore can provide higher resolution but can not provide throughputs competent for mass production. Although the higher and higher beam currents can be used to increase the throughputs, the superior spatial resolutions will be fundamentally deteriorated by Coulomb Effect.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots on one being-inspected or observed surface of a sample. For the sample surface, the plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≦50 eV) and backscattered electrons (energies close to landing energies of the electrons). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than scanning the large observed area with a single beam.
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions by a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. The apparatus therefore is generally called as a multi-column apparatus. The plural columns can be either independent or share a multi-axis magnetic or electromagnetic-compound objective lens (such as U.S. Pat. No. 8,294,095). On the sample surface, the beam interval between two adjacent beams is usually as large as 30˜50 mm.
For the latter, a source-conversion unit is used to virtually change the single electron source into a plurality of sub-sources. The source-conversion unit comprises one beamlet-forming means and one image-forming means. The beamlet-forming means basically comprises a plurality of beam-limit openings, which divides the primary electron beam generated by the single electron source into a plurality of sub-beams or beamlets respectively. The image-forming means basically comprises a plurality of electron optics elements, which either focuses or deflects the plurality of beamlets to form a plurality of parallel images of the electron source respectively. Each of the plurality of parallel image can be taken as one sub-source which emits one corresponding beamlet. The beamlet intervals, i.e. the beam-limit opening intervals are at micro meter level so as to make more beamlets available, and hence the source-conversion unit can be made by semiconductor manufacturing process or MEMS (Micro Electro Mechanical Systems) process. Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of parallel images onto and scan the plurality of small scanned regions respectively, and the plurality of secondary electron beams therefrom is respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus therefore is generally called as a multi-beam apparatus.
In the source-conversion unit 20-1 in FIG. 1A, the image-forming means 22-1 is composed of a plurality of lenses (22_1L˜22_3L). The substantially parallel primary electron beam 2 from one single electron source is divided into the plurality of beamlets (2_1˜2_3) by the plurality of beam-limit openings (21_1˜21_3) of the beamlet-forming means 21, and the plurality of lenses respectively focuses the plurality of beamlets to form the plurality of parallel images (2_1r˜2_3r) of the single electron source. The plural parallel images are typically real images, but can be virtual images in specific conditions if each of the plurality of lenses is an aperture lens. U.S. Pat. No. 7,244,949 and U.S. Pat. No. 7,880,143 respectively propose an multi-beam apparatus with one image-forming means of this type. In the source-conversion unit 20-2 in FIG. 1B, the image-forming means 22-2 is composed of a plurality of deflectors (22_2D and 22_3D). The divergent primary electron beam 2 from one single electron source is divided into the plurality of beamlets (2_2 and 2_3) by the plurality of beam-limit openings (21_2 and 21_3) of the beamlet-forming means 21, and the plurality of deflectors respectively deflects the plurality of beamlets to form a plurality of parallel virtual images (2_2v and 2_3v) of the single electron source.
The concept of using a deflector to form a virtual image of an electron source was used in the famous two-slit electron interference experiments as early as in 1950s, wherein an electron biprism is employed to forms two virtual images as shown in FIG. 2 (FIG. 1 of the paper “The Merli-Missiroli-Pozzi Two-Slit Electron-Interference Experiment” published in Physics in Perspective, 14 (2012) 178-195 by Rodolfo Rosa). The electron biprism basically comprises two parallel plates at ground potential and a very thin wire F therebetween. When a potential not equal to ground potential is applied to the wire F, the electron biprism becomes two deflectors with deflection directions opposite to each other. The primary electron beam from the electron source S passes the two deflectors and becomes two deflected beamlets which form the virtual images S1 and S2 of the electron source S. If the potential is positive, the two beamlets overlap with each other and the interference fringes appear in the overlapping area.
Since then, the foregoing concept has been employed in a multi-beam apparatus in many ways. JP-A-10-339711 and U.S. Pat. No. 8,378,299 directly use one conventional electron biprism to form two probe spots on the sample surface. U.S. Pat. No. 6,943,349 uses one annular deflector (its FIG. 5) or one corresponding deflector array (its FIG. 12) to form more than two probe spots on the sample surface and therefore can provide a higher throughput. The annular deflector includes an inner annular electrode and an outer annular electrode. If the potentials of the two annular electrodes are not equal to each other, one electric field in the local radial direction will appear within the annular gap therebetween, and consequently the annular deflector can deflect more than two beamlets together in different directions. Furthermore, the deflection function of the annular deflector can be performed by one corresponding deflector array which has a plurality of multi-pole type deflectors arranged along the annular gap.
In the conventional source-conversion unit 20-2 in FIG. 1B, due to the divergence of the primary electron beam 2, the plurality of beamlets passes through the plurality of beam-limit openings with different angles of incidence and therefore suffers strong and different electron scatterings. The scattering electrons in each beamlet will enlarge the probe spot and/or become a background noise and therefore deteriorate the image resolution of the corresponding scanned region.
In U.S. Pat. No. 6,943,349, the current of the plurality of beamlets can only be changed by varying either the emission of the single electron source or the sizes of the beam-limit openings. The single electron source takes a long time to become stable when the emission thereof is varied. The beamlet-forming means needs to have more than one group of openings and the sizes of the openings of one group are different from the other groups. It is very time-consuming to change the group in use. In addition, the secondary electron beams can only be focused onto the multiple detection elements of the in-lens detector in some specific operation conditions of the objective lens. Therefore the available applications are limit.
Accordingly, it is necessary to provide a multi-beam apparatus which can simultaneously obtain images of a plurality of small scanned regions within a large observed area on the sample surface with high image resolution and high throughput. Especially, a multi-beam apparatus which can inspect and/or review defects on wafers/masks with high resolution and high throughput is needed to match the roadmap of the semiconductor manufacturing industry.